Gas giant

A gas giant is a
giant planet composed mainly of
hydrogen and
helium and as a result are sometimes known as a failed star since it contains the same basic elements as a
star.
Jupiter and
Saturn are the gas giants of the
Solar System.The term "gas giant" was originally synonymous with "giant planet", but in the 1990s it became known that
Uranus and
Neptune are really a distinct class of giant planet, being composed mainly of heavier volatile substances (which are referred to as "ices"). For this reason, Uranus and Neptune are now often classified in the separate category of
ice giants.

Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass.[1] They are thought to consist of an outer layer of
molecular hydrogen surrounding a layer of liquid
metallic hydrogen, with probably a molten rocky core. The outermost portion of their hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The layer of metallic hydrogen makes up the bulk of each planet, and is referred to as "metallic" because the very large pressure turns hydrogen into an electrical conductor. The gas giants' cores are thought to consist of heavier elements at such high temperatures (20,000
K) and pressures that their properties are poorly understood.[1]

The defining differences between a
very low-mass brown dwarf and a gas giant (estimated at about 13 Jupiter masses) are debated.[2] One school of thought is based on formation; the other, on the physics of the interior.[2] Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced
nuclear fusion at some point in their history.

Terminology

The term gas giant was coined in 1952 by the science fiction writer
James Blish[3] and was originally used to refer to all
giant planets. It is, arguably, something of a misnomer because throughout most of the volume of all giant planets, the pressure is so high that matter is not in gaseous form.[4] Other than solids in the core and the upper layers of the atmosphere, all matter is above the
critical point, where there is no distinction between liquids and gases. The term has nevertheless caught on, because planetary scientists typically use "rock", "gas", and "ice" as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what
phase the matter may appear in. In the outer Solar System, hydrogen and helium are referred to as "gases"; water, methane, and ammonia as "ices"; and silicates and metals as "rock". Because Uranus and Neptune are primarily composed of, in this terminology, ices, not gas, they are increasingly referred to as
ice giants and separated from the gas giants.